What is Clock Skew?

Clock Skew is the difference in arrival time of the same clock signal at two different flip-flops in a digital circuit.

Ideally, the clock should reach all flip-flops at exactly the same time. In reality, due to differences in routing, buffers, wire lengths, and clock tree structure, the clock arrives at different times.

Simple Definition

Clock Skew is the time difference between the clock arrival at the launch flip-flop and the capture flip-flop.

Example

Suppose:

Clock reaches FF1 at 10 ns
Clock reaches FF2 at 10.2 ns

Then:

Clock Skew = 10.2 ns - 10 ns
           = 0.2 ns

So the skew is 200 ps.

Timing Diagram

Launch FF Clock :  ____/‾‾‾\\____
                        ↑
                     10 ns

Capture FF Clock:  _____/‾‾‾\\___
                          ↑
                       10.2 ns

Clock Skew = 0.2 ns

Types of Clock Skew